FIX: stacktrace generation on private mount points in ubuntu

Yama prevented the spawned gdb to ptrace the dying cvmfs process on ubuntu. This only occured for private mount points, because cvmfs cannot re-gain root rights which would override yama-rules.

This uses `prctl(PR_SET_PTRACER, ...)` to create an exception for the spawned gdb. However, this solution is rather complex and could be implemented easier. This requires a proper rewrite of the monitor...